As is known, illuminated panels are increasingly used in cities for advertising purposes or to provide information on traffic, as interior decoration elements, and the like.
Currently, the issues related to illuminated panels concern two very distinct aspects.
First of all, there is the apparent thickness of the device (panel), which is dictated by the light sources located inside the illuminated panels, which must ensure overall uniformity of the illuminated surface of the panel.
The use of fluorescent sources arranged directly below the diffuser (generally a sheet of plastics or an opalescent glass plate) entails that the sources are visible as a bright area, and this effect can be obviated only by moving the diffuser further away, consequently increasing the thickness of the entire panel.
Furthermore, the amount of light emitted can be excessive, indeed because of the need to have a large number of sources to be arranged as uniformly as possible.
The excessive thickness of an illuminated panel or box, especially for indoor use or for application to partitions, shop interiors, etcetera, is not only an aesthetic penalty but also a constructive one.
The second issue relates to the use of the light sources, which substantially range from fluorescent tubes to more recent LEDs. Fluorescent tubes are inexpensive but bulky and have a relatively short useful life.
On the other hand, LEDs are far more compact and expensive, but have an extremely long useful life.
Both light sources suffer a problem in terms of equalization of the illuminated surface of the panel, but the use of LEDs reduces at least the decisive factor of access to the light sources for their maintenance.
One known solution for reducing the thickness of illuminated panels and achieving an energy saving is based on the conversion of the box or panels into hollow bodies with light sources arranged at the inner sides of the panel. The material used for this solution is constituted by a transparent film with microprisms on a single face, which is arranged both on the transparent/opalescent surface of the diffuser and on the back, directly behind the diffuser. Since the orientation of the prisms extends in the direction of the two lateral sources, the light strikes the film with a very shallow angle of incidence and tends to be reflected back toward the inside of the box instead of escaping through the diffuser.
These multiple reflections would lead to no apparent luminosity if one did not intervene, generally in the area that lies furthest from the sources (i.e., the center of the panel), with a light extractor, which converts the multiple specular reflection into a reflection of light in all directions. Extractors are generally constituted by variously configured opaque white surfaces (Lambertian reflection of light).
For example, such surfaces can be constituted by white triangles that become larger as they lie further away from the source, with the goal of equalizing the light reflected in all directions and of having an impression of uniform illumination between the outer regions (near the sources) and the center of the panel.
Another configuration of more sophisticated extractors is constituted by concentrations of opaque white lines that are more or less fixed in relation to the multiple reflection points of the sources, which appear to be mirrored in virtual positions.
However, the solution described above has problems, such as for example the penetration of dust into the device (due to the need for regular maintenance), which greatly reduces its luminous output over time.
The use of LEDs can obviate this drawback, by virtue of the lack of maintenance, as explained earlier, and can also obviate the presence of reflectors behind the fluorescent lamps, which are designed to convey the light at a very shallow angle with respect to the films (the individual LEDs have a lens that already performs this task). However, the cost of transparent films with microprisms on a single face is high, and therefore this is not an attractive solution in terms of cost.
Moreover, devices based on the use of microprismatic films suffer the drawback of requiring duplication of the devices inside the boxes in order to achieve their two-sided illumination, which is increasingly required in environmental signage.